1. Field of the Invention
The present invention relates to a method and device for displaying an image in which a halftone is reproduced by controlling a lighting time per frame and is suitable for a display using a plasma display panel (PDP) or an organic EL panel.
A PDP has two features of high speed and high resolution that are suitable for a television set and a computer monitor. A PDP is used for a large screen display device. One of tasks about a PDP is to reduce a pseudo contour and a flicker of an animation display.
2. Description of the Prior Art
A halftone is reproduced in a PDP by setting the number of discharge times in a frame for a cell (a display element) in accordance with a gradation level. A color display is one type of a gradation display, and a display color is determined by a combination of luminance levels of three primary colors.
As a method of a gradation display using a PDP, a subframe technique is widely known in which one frame is converted into a plurality of subframes having weights of luminance, and the total number of discharge times of one frame is set by combining on and off of the subframes (this is called a lighting pattern). Generally, the conversion from a frame into the subframes is performed by using a conversion table that was prepared in advance. In the case of an interlace display, each of fields making a frame is made of plural subfields, and lighting control is performed for each subfield. However, contents of the lighting control are similar to the case of a progressive display.
In a display with the lighting control in a subframe unit, lighting subframes and non-lighting subframes are mixed, so a timing of the light emission becomes discrete within a frame period. As a result, undesired flickers and pseudo contours may be generated. For example, if the light emission is concentrated in the first half of a display period and is concentrated in the second half of the successive frame of a certain frame, the time period of low luminance becomes long so that the distribution of the light emission along the time axis can be observed as a flicker to eyes of a human being. In addition, when displaying an image including an object that moves within a screen, the image of the noted cell moves on the observer's retina as the observer follows the object by his/her eyes. In this case, if an image of a cell having low light emission intensity stays on a certain point on the retina coincidentally, the surface of the object may be seen in low brightness corresponding to the point. When such points are coupled on a line, a string-like pattern may be observed on the surface of the object, which is called a pseudo contour. In other words, the pseudo contour is a phenomenon that an observer sees a light and dark pattern that is different from the display contents and is apt to be generated especially when an image portion made of pixels having a similar gradation level and a gentle gradient of density moves in a screen. For example, in a scene of a person walking, a portion of his/her face can generate a pseudo contour.
Conventionally, a method for reducing flickers and pseudo contours is known, in which the weighting is performed so that plural sets of subframe expressions can be realized for a halftone, and an optimal subframe expression is selected for each gradation level noting each frame. The basic concept of optimizing the subframe expression is to maintain a barycenter of light emission in a frame period without a substantial change corresponding to the gradation level as disclosed in Japanese unexamined patent publication No. 10-307561. For example, the barycenter of the light emission is set to the middle of the frame period. If the barycenter of the light emission is constant, the interval between the light emission barycenters of the frames also becomes constant, so that uneven distribution of the light emission timing such as a long period of low luminance can be eliminated.
In addition, Japanese unexamined patent publication No. 11-224074 proposes a method comprising the steps of noting a certain frame (i.e., the present frame) for determining a lighting pattern, referring a lighting pattern of a previous frame, and selecting an optimal lighting pattern considering the relationship between the previous frame and the present frame. By this method, the pseudo contours can be reduced more securely than the method in which only the present frame is noted for determining a lighting pattern. In addition, Japanese unexamined patent publications No. 9-172588 and No. 2000-105565 propose a method in which the lighting pattern is determined by considering a lighting pattern of the neighboring cell. By making the lighting pattern change as little as possible among the neighboring cells, the generation rate of the pseudo contours can be reduced.
As explained above, there was a problem that if the lighting pattern is determined by noting only the position of the barycenter, quality of the display depends on an extension of the light emission waveform. For example, if two light emission waveforms having the same barycenter as shown in FIGS. 21A and 21B appear alternately as shown in FIG. 22, the luminance is modulated with the period twice the frame period, and flickers can be observed even if the barycenter position is fixed. Furthermore, it is not sufficient for suppressing pseudo contours to control only the barycenter position. The change of the display when an object moves on the screen is show in FIG. 23. FIG. 23 shows a contour portion of an object having P gradation level moving on a background having Q gradation level. When a line of sight follows the movement of the object, the image of the object stays on a retina, and the quantity of entering light on the retina is distributed as shown in FIG. 24. Considering the lighting pattern, the integral quantity of light on the retina becomes as shown in FIG. 25. It is usual that there is a gap between the cells in a real cell arrangement. For example, a partition for defining cells forms a cell gap. In a color display using three color cells, there is a cell of another color between cells of a certain color, so that a gap corresponding to two cells can be generated. Considering this fact, the quantity of entering light on the retina when there is a cell gap is shown in FIG. 25. FIG. 25 shows the integral quantity of light in one frame made of plural subframes (denoted by SF in FIG. 25). Though there is no overlap of light emission profiles between the neighboring cells in FIG. 25, but it is not essential. The presence or absence of the overlap depends on a moving rate of the line of sight and a lighting pattern.
Usually, the observation of the screen is performed in the state where a cell pitch is smaller than the resolution of eyes. Therefore, light quantity profile on the retina shown in FIG. 25 is observed as being averaged in the space direction. In the display error that is a shift from the target light quantity, components having spatial frequencies above the cell pitch are not recognized. Components having spatial frequencies below the cell pitch mainly contributes to the pseudo contour, while the variation of sparse and dense of the space light emission profile corresponds to a dark portion and a bright portion. The variation of sparse and dense of the light emission profile cannot be controlled only by the barycenter of the light emission and is affected by the expansion of the lighting pattern as shown in FIGS. 26 and 27. In FIG. 26, the light emission is concentrated in the middle portion of the projection area of the frame in both of the neighboring cells, so the pseudo contour is not conspicuous. In contrast, though the pattern shown in FIG. 27 has the pattern of the barycenter of the light emission equal to the pattern shown in FIG. 26, the light emission of one of the cells is unevenly distributed to the edge portion of the projection area of the frame. In this case, the variation of sparse and dense of the light emission intensity may occur between the neighboring cells and be observed as a pseudo contour.
As explained above, from the viewpoint of the pseudo contour, the optimal lighting pattern is not always selected only by aligning barycenter positions. In addition, the conventional method in which only the individual cell is noted so as to make the lighting pattern vary little between the present frame and the past frame is not sufficient for reducing flickers and pseudo contours.
Moreover, in the conventional method, it is necessary that a skilled person decide in accordance with experiences which lighting pattern should be selected for each gradation level when making a conversion table that connects a frame with subframes. If the relationship between the previous frame and the present frame is considered as explained above and the gradation number N is supposed to be 256, an optimal lighting pattern must be determined for each of 2562 gradation levels with enormous efforts. If two or more previous frames are referred, the combination of the gradation levels becomes up to N3. When the gradation number N is increased or the weight is changed, the change of the specification causes tiresome jobs every time.